1. Field of the Invention
The invention relates to the field of fluid data systems and, in particular, to a hydrodynamic air data system for use on aircraft and the like.
2. Description of Related Art
The typical aircraft air data system uses pitot tubes that measure both dynamic and static pressure and calculates calibrated airspeed, Mach number, and barometric pressure altitude there from. Expressing the speed of sound as a function of only the square root of the absolute temperature, one needs only a gauge measurement of temperature for its calculation. From the Mach number and the calculated speed of sound, the true airspeed can be computed. However, pitot tubes must extend out from the fuselage of the aircraft. Thus they tend to increase the radar cross-section (RCS). On stealth aircraft, such as the F-117A, extensive shaping of such pitot tubes along with the application of expensive radar absorbing coatings are equipped to reduce the aircraft RCS to acceptable levels. Pitot tubes must also incorporate heaters to prevent the ports from icing. On extremely high speed flight, such as the space shuttle experiences on reentry, the pitot tubes are retracted to prevent damage from the intense aerodynamic heating and, thus, are kept retracted until the speed is reduced to around Mach 3. Flush mounted systems based on the use of lasers have been developed to make such measurements, however, they are both expensive and heavy.
Another critical measurement necessary for control of an aircraft is the angle between the aircraft longitudinal axis and the relative wind (angle of attack and angle of slip). The typical system for measuring these angles uses an external probe. The probe includes four pressure ports equidistant about the circumference thereof, two aligned with the vertical axis to measure angle of attack and the other two aligned with the horizontal axis for yaw measurement. If the aircraft is at any angle to the relative wind, the pressure measurement obtained by the two aligned ports will differ. Of course, this difference will be proportional to the angle of attack or angle of slips. However, it is obvious that such a probe has the same drawbacks as the pitot tube.
Active acoustic air data systems, which can provide airspeed and angle of attack, are also old in the art. Most are based upon the principle that the transmission time of sound waves in a fluid along a given path is a function of the sum of the local acoustic velocity plus the local fluid velocity component parallel to that path. If the fluid velocity is in the same direction as the direction of propagation of a sound wave, the transmission time of the sound wave between two points of given separation is minimum. Similarly, if the fluid velocity has a direction opposite to the direction of propagation of a sound wave, the transmission time of the sound wave between the two points is maximum. Accordingly, if sound waves are propagated in a fluid along a plurality of non-parallel paths, each path having the same length and each being coplanar with each other and with the direction of fluid velocity to be measured, the transmission times of the sound waves along each of the paths will vary in accordance with the fluid velocity and direction; i.e., the magnitude of the fluid velocity component along each path. However, all such systems are based on the concept of transmitting an acoustic signal (sound waves) by means of an electromechanical transducer (e.g., a sound source) through the fluid medium to one or more receivers and measuring the travel time to each downstream array receiver.
An example of such a system is found In U.S. Pat. No. 3,379,060 "Wind Meter" by C. B. Pear, Jr. A first electroacoustic transducer is provided for transmitting a pulse of sound along a plurality of non-parallel paths which are co-planar with the direction of a fluid velocity to be measured. A plurality of second electroacoustic transducers, one for each of the plurality of paths, are spaced concentrically about the first transducer and along the plurality of paths, for receiving the sound pulse transmitted by the first transducer. Associated with each receiving transducer is an indicating means which is energized when a pulse of sound arrives. There is also provided logic circuitry so that when a pulse is received by one of the receiving transducers, only its indicator is actuated and all other indicators are inhibited from operating until the logic circuit is reset. Since there will be only one path which is essentially parallel to the fluid flow velocity direction, the receiving transducer associated with that path will receive the sound pulse before it is received by any of the other receiving transducers, actuating its indicating means to the exclusion of all other indicating means. By observing which indicating means is activated, a measurement of fluid flow direction is obtained.
In order to determine fluid velocity, the first received sound pulse is used to generate a new pulse which drives the transmitting transducer after a predetermined fixed delay which is sufficient to allow the preceding sound pulse to have reached all of the receiving transducers under all conceivable weather conditions. By so driving the transmitting transducer, the pulse repetition rate will be a function of the minimum transit time of a sound pulse traveling from the transmitting transducer to one of the plurality of receiving transducers, which time is, of course, directly related to the fluid velocity and speed of sound. The fluid flow velocity is obtained directly by measuring the minimal pulse arrival time difference between the acoustical source and the downstream receiver transducers.
In U.S. Pat. No. 4,143,548 "Measuring The Speed Of An Aircraft" by E. Graewe, et al. a continuous ultrasonic wave transmitter transmits modulated waves in two opposite directions which are intercepted by a forward receiver and an aft receiver. The phase differences between the transmitter signal and receiver signals are used to calculate transit time differences from which the received signals are demodulated and the relative phase is used to calculate the speed of the aircraft. A non-zero angle between the longitudinal axis of the aircraft and the actual direction of propagation, can be compensated by including two orthogonally arranged receivers to obtain a speed vector (angle of attack).
A third method is disclosed in U.S. Pat. No. 4,112,756 "Ultrasonic Air Data System" by P. H. B. MacLennan, et al. This ultrasonic air data system determines the relative velocity of an aircraft with respect to the medium in one, two or three directions. In addition, it may determine the speed of sound and the approximate temperature. In the one direction system, a first ultrasonic transducer transmits a pulse to a second transducer where it is detected and reflected back to the first transducer and again reflected to the second transducer. The pulse transmit times for each direction are determined, and the relative velocity, the speed of sound and the approximate temperature are provided as a function of the transit times. In the two or three direction systems, three or four transducers are positioned in a two or three dimensional configuration. In the first half cycle, the first transducer transmits a pulse to the second where it is reflected to the third or last transducer in a two-dimension system and then to the last transducer in a three-dimension system. In the second half-cycle, the last transducer transmits a pulse which is reflected through the transducer arrangement to the first transducer. Transit times for pulse travel in each direction between pairs of transducers are determined, and relative velocities, speed of sound and approximate temperature are provided as a function of these transit times.
In all three of the above systems an ultrasonic transducer or "loud speaker" is required. All of the following acoustic systems require ultrasonic transducers or loud speakers: U.S. Pat. No. 4,708,021 "Arrangement For Contactless Measurement Of The Velocity Of A Moving Medium"--by H. Braun, et al., U.S. Pat. No. 5,040,415 "Nonintrusive Flow Sensing System" By S. Barkhoudarian, U.S. Pat. No. 4,484,478 "Procedure And Means For Measuring The Flow Velocity Of A Suspension Flow, Utilizing Ultrasonics" By E. Harkonen, U.S. Pat. No. 4,112,756 "Ultrasonic Air Data System" By P. Barry, et al., U.S. Pat. No. 4,995,267 "Method of Monitoring The State Of Elongated Object And Apparatus For Performing This Method" By S. Mikheev, et al. U.S. Pat. No. 4,351,188 "Method And Apparatus For Remote Measurement Of Wind Direction And Speed In The Atmosphere" By M. Fukushima, et al. U.S. Pat. No. 4,831,874 "Paradac Wind Measurement System" By S. Daubin, et al., U.S. Pat. No. 4,468,961 "Fluid Direction Meter Suitable For Angle Of Attack Meter For Aircraft" By L. Berg, U.S. Pat. No. 4,611,496 "Ultrasonic Flow Meter" By T. Komachi, U.S. Pat. No. 3,548,653 "Direction And Velocity Determining Apparatus" By V. Corey, U.S. Pat. No. 4,576,047 "Apparatus For Determining The Transit Time Of Ultrasonic Pulses In A Fluid" By R. Lauer, et al., U.S. Pat. No. 4,174,630 "Ultrasonic Anemometer" By J. Nicoli, U.S. Pat. No. 3,693,433 "Ultrasonic Anemometer" By Y. Kobori, et al., and U.S. Pat. No. 4,043,194 "Wind Shear Warning System"--By J. Tanner.
Thus it is a primary object of the subject invention to provide a hydrodynamic data system for a vehicle such as an aircraft and the like that is flush with the surface of the vehicle.
It is another primary object of the subject invention to provide a hydrodynamic air data system for a vehicle such as an aircraft and the like.
It is a further object of the subject invention to provide an acoustic air data system for a vehicle such as an aircraft and the like that uses only passive .acoustic sensors.
It is a still further object of the subject invention to provide a hydrodynamic air data system for a vehicle such as an aircraft and the like that can provide true air speed, Mach No., absolute temperature, barometric altitude and angle of attack.